I'd like the ranging shot firing order to be optimized on ships like renommee or st pavel; the upper deck guns are annoying to have because if you have them equipped, they fire first by default and make aiming the entire broadside difficult instead of adding to it. To avoid this you can disable them, but keeping track of when they are loaded and if you want them to fire makes them annoying to micromanage for a really small increase in firepower. I would like the firing order for ships with weird configurations to be tweaked so that they fire in a pattern that is more useful to aim or range a larger broadside. For instance on the pavel, have the 2nd from top deck of guns fire first with ranging shots, and only the top deck guns fire when the natural progression of shots from the lower deck reaches that point along the ship.
Crudely drawn example of gun layout for a ship like renommee:
Currently the game would fire in this order:
8, 9, 1, 2, 3, 4, 5, 6, 7.
I would prefer either
1, 2, 3, 4, 5, 6, 7, 8, 9.
or better
1, 2, 3, 4, 8, 5, 9, 6, 7.
A small change, but one that would make the quality of life better for any ship with a huge gap in its upper deck or very few guns there.

A small ship delaying a big ship in real life does not get to take advantage of time compression. If a prince attacked a bellona the bellona might not even beat to quarters, just have one or two gun ports open to make sure the prince stays away. It could continue on its mission quite normally. In NA, the light vessel can just tag them and wait while the Bellona is trapped in slowed down time and the approaching revenge fleet travels at warp speed relative to the victim. This is unrealistic and exploitative.
Time compression makes delay/immobilize tagging so overpowered when small ships do it to large ones; and as we cannot remove time compression it is the non-aggressive tagging and running away that must be dealt with.

A lot of people are saying that before we continue to work on balance between ship classes, we should fix the economy. I think that until the economy is top notch and balanced, ship classes cannot be balanced in a satisfying way. Naval warfare wasn't team fortress 2: different classes of ship were NOT balanced in a 1v1 or even any fighting situation. If a frigate saw a 1st rate, they kept their distance or took one or two cheeky shots at the stern. People still made frigates because lineships were ludicrously expensive and not practical for protecting trade or suppressing piracy. In naval action, there is no economic reason to not use lineships because players can farm up the resources for them relatively quickly, and as others have stated the difference between ship classes is linear not exponential. In addition fir/fir lineships can still catch fully laden traders, so frigates are even less useful.
As a result, if we make lineships as lethal as really were, nobody uses anything else for serious PVP. So lineships are nerfed to allow some diversity in gameplay, which means they are frustrating and unrealistic to use. To return to the tf2 analogy, lineships are like the heavy while frigates are like the soldier or scout. The specifics of the analogy aren't important, what's important is that the trade-offs are relatively fair. Lineships' increased damage correlates "fairly" to their lack of mobility. This works balance wise because they are all available relatively easily to players willing to grind a little; however, its not very satisfying for the lineships and first rates to get beaten by smaller ships because of hugging, sterncamping, etc etc. The key to balancing them is to add the historical balancing factor: cost. This requires a difficult game design decision and a very refined, difficult to exploit economy where players cannot garner obscene amounts of wealth very quickly.
Early on in the development of the game I remember the question of ship cost and rarity was discussed, and the following conclusion was arrived at: every player should be able to play the ship they want, if they are willing to put a reasonable amount of effort towards crafting or buying it. I believe this is a mistake with unavoidable ramifications for game balance and realism.
Players being able to play whatever ships they want
Lineships being satisfying and realistic to sail.
An vibrant open world full of all ship sizes and classes.
The above three goals cannot be achieved simultaneously. You can have at best two:
If players can access any class of ship easily and pvp is to feature a diverse range, then lineships must be unrealistically nerfed to make them fair fighting alongside smaller vessels. (stern camping, hugging, shot damage based on diameter not weight.)
For pvp to be diverse and lineships satisfying, they must cost too much for most players to acquire. (my preferred scenario)
If line-ships are available to all and realistically powerful vs other classes, very few players will sail frigates or smaller lineships and consequently fleet composition will be uniform/stagnant. (thickness meta best meta, 25 LOWO oceans vs 25 LOWO oceans)
As someone who rarely sails lineships in favor of brigs and corvettes, I know that making 1-3 rates an extremely rare clan wars or hyper rich trader only option is easy for me to suggest but would be a lot more painful for solo players who really want to sail 1st rates, but as I argued above, we can't have it all here. A full fleet of 25 LOWO oceans should be so mindbogglingly valuable that no sane clan would risk them except for the most absolutely important PB imaginable. Sailing a lineship out into the OW alone and un-escorted should be crazy. For a normal solo player like me, a medium to large frigate or maybe 4th rate should be the maximum ship I can comfortably buy, equip, and insure. The big problem with making cost a balancing factor is that it doesn't affect all players equally. Some have found ways to grind currency so efficiently that cost is meaningless to them. To balance ships based on cost and have it actually effect how players sail and what ships they command, the economy must be more clamped down and tougher, with fewer get rich quick techniques and farming methods. In addition if RVR becomes "who can buy the most 1st rates wins every time" that would defeat the purpose and do more damage than good. It's an incredibly complex topic.
But anyway these opinions are probably quite outdated, I haven't played the game too much in the past year.

Single player Naval Action could be just as incredible as the multiplayer game. The level of detail in combat but with realistic map sizes and time compression at the player's will, less concessions towards pvp balance, missions/quests, more realistic progression within the navy/trading company/pirates, specialization, single player focused trade, etc. When I first started playing NA, that is the game I wanted (I didn't realize how incompatible my ideal game is with the MMO nature) Obviously Naval Action is an online multiplayer game, but one day after Naval Action is more or less done, I hope Game Labs considers the possibility of single player naval game sharing much of the combat module with NA.
As for the whole "pve server vs pvp server" thing, I suggest making the Caribbean a PVP zone and the Pacific a PVE/exploration zone. Assets and characters unable to be transferred between the two in general, but with some Pacific only loot and Caribbean only loot trade-able between the two, allowing some interaction between the two groups and an unusual player driven trade option. That way everyone is on the same server and can chat to each other while still playing the game their own way. I'm unaware of if this is even technically possible however.

I would rather see the binary nature of boarding addressed than have triggering boarding easier. Going from sailing along next to each other to sails dropped full stop instantly isn't a realistic solution.
The crew don't wait till both ships are completely lashed together to start fighting. Perhaps have ships in close proximity to each other up a small window to the right of the screen that allows some boarding commands while the two ships are still manouvering and sailing. For instance musket fire and fire deck guns (swivels) can be used any time the enemy is near, if crew are in boarding and enough prep is available. Grenades is possible at extremely close range, and attack requires the two to be lashed together.
Some boarding commands can be tweaked, for instance "musket volley" can consume prep and damage crew constantly while active. Same with grenades, while "fire deck guns" would depend on the actual deck gun reload state.

I'll say it again, people will be perfectly willing to purchase DLC even if it offers no game play avantage.
In Team Fortress 2 people pay real money to be able to turn themselves into pigeons and cover their rocket launchers in christmas lights. I'm sure they will pay for bow figures, sail patterns, and paint jobs.

We've probably all heard the story/fable of the guy hitting himself in the head with a hammer "because then it will feel good when I stop!"
That is skillbooks in NA. Getting skillbooks only feels fricken awesome because of how annoying all the time before getting them is.